1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing thereof. Particularly, the present invention relates to a method of forming on a surface of a silicon semiconductor substrate, an insulating film unlikely to cause a dielectric breakdown, as well as to a semiconductor device having the above stated insulating film.
2. Description of the Prior Art
FIG. 1A is a sectional view showing schematically a structure of a conventional semiconductor device of metal-oxide-semiconductor (MOS) type. In FIG. 1A, a semiconductor device, that is, a MOS capacitor for example comprises a silicon semiconductor substrate 1, a silicon oxide film 2 formed on the silicon semiconductor substrate 1 by a thermal oxidation method and an electrode layer 3 formed of polysilicon for example. The silicon oxide film 2 serves as a dielectric layer.
FIG. 1B is a sectional view showing schematically a structure of another conventional semiconductor device of MOS type. The semiconductor device shown in FIG. 1B comprises: a silicon semiconductor substrate 1; an impurity diffusion layer 4 formed by diffusion of an impurity such as boron, arsenic or phosphorus in a surface of the siliocn semiconductor substrate 1; a silicon oxide film 2 formed on the impurity diffusion layer 4 by thermal oxidation of the impurity diffusion layer 4; and an electrode layer 3 formed on the silicon oxide film 2. The silicon oxide film 3 serves as a dielectric layer between the electrode layer 3 and the silicon substrate 1 or between the electrode layer 3 and the impurity diffusion layer 4. It is generally said that an electric field effecting a dielectric breakdown on a silicon oxide film formed by a thermal oxidation method is approximately 10 MV/cm. However, with an electric field of less than 10 MV/cm, a dielectric breakdown of a silicon oxide film would also be caused as the electric field applying time proceeds. Such dielectric breakdown phenomenon in a dielectric film caused with the passage of the electric field applying time is called TDDB (Time Dependent Dielectric Breakdown). The mechanism of the TDDB has not been clarified so far.
In the operation of such a semiconductor device as shown in FIG. 1A or FIG. 1B, a predetermined electric field is applied between the electrode layer 3 and the silicon substrate 1 or the impurity diffusion layer 4 so that the silicon oxide film 2 is exposed to the electric field. If a TDDB phenomenon occurs in the silicon oxide film 2 by the application of the electric field, insulation defective is effected between the electrode layer 3 and the silicon substrate 1 or the impurity diffusion layer 4, resulting in a malfunction of the semiconductor device. As described previously, in the MOS type semiconductor devices, a silicon oxide film formed on the surface of the silicon substrate with thermal oxidation of an impurity diffusion layer is used as an insulating film (or a dielectric film). However, in a conventional device, no consideration is given to the relation between the quality or the concentration of the impurity in the underlying impurity diffusion layer and the time elapsed to cause a TDDB in the silicon oxide film formed by the thermal oxidation. Consequently, a disadvantage is involved in that the time elapsed until a dielectric breakdown occurs in the silicon oxide film is sometimes shorter than an estimated value in design (or in specification). A relation between the dose of impurity implanted in an impurity diffusion layer and the endurance of a silicon oxide film is disclosed by S. Morita et al. in the Proceedings, l4a-B-9, the 1984 Autumn Meeting of Japan Applied Physics.